Spraying method to form a thick coating and products obtained

ABSTRACT

A method and an apparatus for spraying materials onto a substrate to produce a coating thereon is described which allows very thick layers of complex metal oxides to be produced. The apparatus and method are particularly suitable for producing superconducting coatings.

The present invention relates an apparatus and a method of spraying toform a coating on flat or curved substrates, for example, either as partof the direct formation of metallic or ceramic coatings such assuperconductive or piezo-electric layers or for the production oftargets for sputtering magnetrons having coatings which are precursorsof such layers.

TECHNICAL BACKGROUND

From EP-A-286 135 it is known to flame spray complex ceramic materialsonto a substrate such as a tape to form a superconducting layer. It issuggested to pre-heat the substrate to temperatures above 540° C. and tocool the coating slowly. It is further recommended to treat the coatingin an atmosphere containing one of the components of the superconductingceramic. An oxy-acetylene flame is used for the flame spraying.Thickness of up to 3 mm are described.

It is also known from U.S. Pat. No. 5,196,400 to plasma spray a coatingonto a target for use in a sputtering magnetron to sputter a Y—Ba—CuOsuperconductor coating. Deposition of only a thin target coating of 0.5mm is reported.

The production of superconducting powders using flame spraying isreported in U.S. Pat. No. 5,140,005. An oxy-acetylene flame is used. Itis tacitly accepted that the high temperature of the flame changes thestoichiometric ratios of the components and that this has to becompensated by increasing the more volatile components in the originalmixtures. U.S. Pat. No. 5,045,365 describes a method of cooling aoxy-acetylene flame-sprayed substrate with water. Without specialprecautions, water cooling is unsuitable for superconductors due to thewater vapour produced.

EP-A-355 736 describes production of flat targets with metal oxides upto a layer thickness of 3 mm. WO 98/0833 describes the production of <20micron thick layers of superconducting metal oxide mixtures.

The article by Murakami et. al. “Rapidly Solidified Thick Deposit Layersof Fe—C—Mo Alloys by Flame Spraying” describes up to 1.5 mm thickrapidly cooled thick layers of Fe—C—Mo alloys by flame spraying. Specialprecautions were taken to produce dense layers, e.g. direct applicationof cryogenic gas on the coating during application.

EP-A-586 809 describes the metal spraying application of a layer ofrelatively homogeneous material (nickel coated silicon) which is mucheasier to handle than the heterogeneous oxide mixtures contemplated bythe present invention. Layer thicknesses of up to 8 mm are described but3 to 5 mm is preferred. Various layers are proposed including a Ni—Allayer for improving adhesion between the deposited layer and thesubstrate. A Ni—Al adhesion promoter is known from DE-A-33 18 828.

Plasma spraying of superconducting materials is described in EP-A-288711 up to a thickness of 250 micron.

It is an object of the present invention to provide an apparatus and amethod of spraying heterogeneous metal oxides to form a ceramic coatingon flat or curved substrates.

It is a further object of the present invention to provide an apparatusand a method of spraying heterogeneous metal oxides to form a thickwalled ceramic coating on flat or curved substrates which isstructurally sound.

It is a further object of the present invention to provide an apparatusand a method of spraying to form a thick walled coating of asuperconducting ceramic material.

It is still a further object of the present invention to provide anapparatus and a method of spraying suitable for forming a thick walledceramic coating on flat or curved targets to be used in a sputteringmagnetron.

It is still another object of the present invention to provide a methodof producing a (magnetron) vacuum sputtering target as well as thetarget itself with improved thermal and electrical conductivity and highmechanical strength using a spraying process employing dedicated powderformulations.

SUMMARY OF THE INVENTION

One aspect of the present invention is to provide a substrate with acoating of a combination of metal oxides having a thickness greater than3 mm more preferably greater than 5 mm and most preferably greater than8 mm. Preferably, the coating is deposited by spraying, e.g. flame orplasma spraying. Preferably, the substrate is cylindrical and is morepreferably is suitable as a cylindrical target substrate for asputtering magnetron. The combination of oxides preferably comprises atleast a superconductive precursor or a superconductor. The thermalconductivity of the deposited material is preferably between 1 and 5Wm⁻¹K⁻¹. When deposited on a steel substrate the thermal conductivity ofthe composite preferably lies within the range 25 to 125 Wm⁻¹K⁻¹. Thesevalues are particularly preferred for YBa₂Cu₃O₇ coatings. Preferably, anadhesion promoter layer is applied onto the substrate before applicationof the coating of the metal oxide combination. The adhesion promoter maybe a layer of Ni—Al or a layer of an In-alloy, for example. Thedeposited coating is preferably impact resistant, e.g. withstands impactof a 0.036 kg steel ball from a height of 2 meters. Preferably, about20% or up to 30% of a noble metal is included in the oxide material toimprove electrical and thermal properties of the deposited layer. Thenoble metal is preferably silver. The noble metal may in included as asalt or oxide, e.g. silver nitrate or silver oxide, in the material tobe sprayed. Preferably, the electrical resistivity of the depositedlayer is lower than 15×10⁻⁶ Ohm.m, more preferably lower than 10×10⁻⁶and most preferably less than 5×10⁻⁶ Ohm.m. Values below 1×10⁻⁶ Ohm.mcan be achieved. Up to 30% of a noble metal such as silver may be addedto lower the resistivity. These values are particularly preferred forYBa₂Cu₃ 0 ₇ coatings.

The electrical, thermal and mechanical properties of the coatingdeposited in accordance with the present invention should be sufficientthat the deposited layer can be applied to a suitable substrate by meansof a sputtering magnetron preferably at a static sputtering depositionspeed of at least 5 nm/minute, more preferably, at 20 nm/minute and mostpreferably at at least 40 nm/minute.

When a superconductor precursor or a superconductive material isdeposited, at least 10% of the coating is in the superconducting phase,more preferably 15%. This may be assisted by a subsequent limitedthermal treatment, e.g. 3 hours and 940° C., after deposition.

The present invention also includes a method of depositing by spraying asuperconductor precursor layer onto a cylindrical target for asputtering magnetron, the layer having a thickness of at least 3 mm, andat least 10% of the layer being in a superconductive phase. The presentinvention also includes a method of depositing by spraying a layer ontoa substrate, the layer having a thickness of at least 5 mm, and thecoating comprising metal oxides.

In accordance with one aspect of the present invention a flame sprayingapparatus is provided for depositing a metal oxide combination onto asubstrate to produce a coating thereon, comprising: a burner forproducing a flame; an inlet for feeding material to be sprayed throughthe flame, the flame imparting a temperature to the material to besprayed of 1500° C. or less, preferably 1200° C. or less. Preferably thetemperature imparted may be a little higher than the melting point ofthe powder to be sprayed, e.g. 600 to 1000° C. for some metal oxides.Preferably, the thickness of the deposited coating is greater than 3 mmmore preferably greater than 5 mm and most preferably greater than 8 mm.

Another aspect of the present invention is to provide a flame sprayingapparatus for depositing a metal oxide combination onto a substrate toproduce a coating thereon, comprising: a flame spraying gun; and acooling system for the substrate, the cooling system including a devicefor bringing a cryogenic fluid into contact with the substrate.Preferably, the thickness of the deposited coating is greater than 3 mmmore preferably greater than 5 mm and most preferably greater than 8 mm.The input material for the sprayer may be a liquid solution of solublecompounds (e.g. nitrates) which decompose thermally into ceramiccomponent oxides, liquid slurries of the ceramic components or metalpowders, or dry metal or ceramic powders or precursors of the ceramiccomponents, e.g. nitrates, of such powders.

The present invention may provide a method of flame spraying acombination of metal oxide materials onto a substrate to produce acoating thereon, comprising: generating a flame; feeding the material tobe sprayed through the flame, the flame imparting a temperature to thematerial to be sprayed of 1500° C. or less, preferably 1200° C. or less.Preferably the temperature imparted may be a little higher than themelting point of the powder to be sprayed, e.g. 600 to 1000° C. for somemetal oxides.

The present invention may also provide a method of flame spraying metaloxide combinations onto a substrate to produce a coating thereon,comprising: generating a flame for spraying the materials; and coolingthe substrate by bringing a cryogenic fluid into contact with thesubstrate.

The present invention may also provide a method of flame spraying asuperconducting ceramic material or a precursor thereof onto a substrateto produce a coating thereon, comprising: generating a flame forspraying the ceramic material; depositing the coating on the substrate;and during deposition of the coating, cooling the substrate so that thesolidified coating thereon has a temperature between room temperature(˜25° C.) and 150° C., preferably room temperature (˜25° C.) and 100° C.Water or cryogenic fluid cooling are particularly preferred.

One linking concept between the above methods and apparatus is controlof the total heat energy into the spraying/coating system. This can beachieved by careful control of parameters which influence the energyinput such as spraying distance, spray head traverse speed, rotationspeed of a cylindrical substrate, powder dwell time in the hot exitplume from the spray head, particle velocity exiting the spray head,cooling method and rate of cooling the substrate during coatingdeposition.

The present invention also includes a method of reconditioning a targetfor a sputtering magnetron by flame spraying or atmospheric plasmaspraying as well as a reconditioned target as made in accordance withthe method. The target material or coating is preferably a ceramiccoating, in particular a superconducting or superconductor precursorcoating.

The final coating is preferably a metallic or ceramic layer, inparticular a superconducting or piezo-electric layer or a precursorthereof. The present invention includes a method of spray drying aliquid to form a powder suitable for flame spraying. The spray driedpowder may be sintered. The present invention also includes amanufacturing method for depositing a coating on a substrate comprisingthe steps of: spray drying a precursor liquid to form a powder and flamespraying the powder to form a coating on a substrate. The substrate maybe a target for a sputtering magnetron and the final coating maysputtered onto a final substrate in the sputtering magnetron. Theceramic powder may be sintered after the spray drying step. The flame ofthe flame spray gun preferably imparts a temperature to the powder to besprayed of 1500° C. or less, preferably 1200° C. or less. Preferably thetemperature imparted may be a little higher than the melting point ofthe powder to be sprayed, e.g. 600 to 1000° C. for some metal oxides.During flame spraying the target is preferably cooled by bringing acryogenic fluid into contact with the target. In particular the coolingdevice should maintain the solidified coating at a temperature betweenroom temperature (˜25° C.) and 150° C., more preferably between roomtemperature (˜25° C.) and 100° C.

The present invention includes an apparatus for spray drying a liquid toform a powder suitable for flame spraying. The present invention mayalso include an apparatus for depositing a coating on a substratecomprising: a spray drier for drying a precursor liquid to a powder, anda flame sprayer for flame spraying the powder to form a coating on asubstrate. The substrate may be a target for a magnetron. Additionally,a sputtering magnetron for sputtering the final coating onto the finalsubstrate using the target may be provided. The flame of the flame spraygun preferably imparts a temperature to the powder to be sprayed ofslightly above the melting point of the sprayed material. Preferably thetemperature imparted is 1500° C. or less, preferably 1200° C. or less.Temperatures of 600 to 850° C. may be suitable for some metal oxides. Inthe flame sprayer a cooling system for the target is preferablyprovided, the cooling system including a device for bringing a cryogenicfluid into contact with the target. In particular the cooling deviceshould maintain the solidified coating at a temperature between roomtemperature (˜25° C.) and 150° C., more preferably between roomtemperature (˜25° C.) and 100° C.

The above methods may be used, for example, either as part of the directformation of superconductive or piezo-electric layers on the substrate,e.g. a tape, or for the production of coatings on targets for use in asputtering magnetron to sputter a superconducting layer onto a finalsubstrate. The present invention may provide oxide sputtering targetssupporting very high power dissipation thus enabling high sputterdeposition rates of at least 50 nm/min.

The dependent claims describe additional individual embodiments of thepresent invention. The present invention will now be described withreference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a flame spraying apparatus inaccordance with one embodiment of the present invention.

FIG. 2 is a schematic representation of a flame spraying apparatus inaccordance with another embodiment of the present invention.

FIG. 3 is a schematic representation of a spray drying apparatus inaccordance with another embodiment of the present invention.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

The present invention will be described with reference to certainspecific embodiments and with reference to certain specific drawings butthe invention is not limited thereto but only by the claims. Inparticular, the present invention will mainly be described withreference to the deposition of a superconductor precursor orsuperconductive coatings but the invention is not limited thereto butmay be used advantageously with other heterogeneous coatings such asceramic coatings, particularly those having special properties such aspiezo-electric coatings and in particular coatings which containcomponents which can be degraded by high temperatures or which are morevolatile than other components. More particularly the present inventionwill be described with reference to the manufacture of YBa₂Cu₃O₇superconducting powders and coatings but the invention is not limitedthereto but only by the claims. Further one way of carrying out thepresent invention will be described with reference to low temperatureflame spraying but the present invention is not limited thereto. Bycarrying out the invention in accordance with the processing details andprinciples described below thick layer (greater than 3 mm, morepreferably greater than 5 mm and most preferably greater than 8 mm)metal oxide combination coatings suitable for use as a sputteringmagnetron target have been applied by oxy-acetylene flange spraying withwater cooling or by atmospheric pressure or low-pressure plasma sprayingto substrates including cylindrical substrates used in rotating cathodemagnetrons. During plasma spraying gasses may be used such as argon ormixtures of argon and other gasses to shield the plasma spray. Also thepresent invention will mainly be described with reference to an input tothe flame spraying head of spray dried powder. The present invention isnot limited thereto but includes other forms of input materials such asa mixture of the metal oxides, including slurries thereof or mixtures ofprecursors of metal oxides such as metal nitrates as well as slurriesand solutions thereof.

FIG. 1 is a schematic diagram of the flame spraying apparatus 10 inaccordance with a first embodiment of the present invention. A flamespraying gun is represented schematically at 12. The gun 12 may be acommercially available flame spraying gun as for instance available fromSulzer Metco, Westbury, N.Y., USA or a high velocity oxy-fuel sprayinggun available from the same company. The gun 12 may be provided with anair pincher. The gun 12 may be fed with fuel gas in pipe 22, oxygen inpipe 23 and gun cooling air in pipe 24. Additional gases may be suppliedto the gun 12 as described for instance in U.S. Pat. No. 5,273,957 orEP-A413 296. Material to be coated is fed in powder or liquid form, e.g.a dry powder, a slurry of the powder and a liquid or in solution, to thegun via conduit 26 from hopper 21. Gun 12 is mounted on a drive (notshown) which provides the necessary movements of the gun 12 to coat thesubstrate 19. When substrate 19 is a cylindrical target, for instance,for a rotating cathode magnetron, this may be rotated and the movementsof the gun 12 may be simple reciprocating movements parallel to the axisof the target 19. If the substrate 19 is a flat rectangular or circularplate, the movements may be provided by a suitable robot and may becomplex, e.g. including rotational cycloidal motions. For rapiddeposition several guns 12 may spray the same substrate 19 at the sametime.

The fuel gas for the gun 12 may be selected from one of acetylene,propylene, hydrogen or similar fuels but the present invention is notnecessarily limited thereto. Particularly preferred in one embodiment ofthe present invention is a fuel with a lower calorific value such as oneof ethylene, natural or town gas, butane or propane as these provide alower temperature flame than acetylene and butane is particularlypreferred as it gives a stable easily controllable flame and isconsidered safer than acetylene if powders containing copper compoundsare used. It is generally accepted that oxy-acetylene flames havetemperatures of 2000° C. and more. It is preferred in accordance with anembodiment of the present invention if the flame of the flame sprayinggun 12 imparts a temperature only sufficient to just melt the powder tobe sprayed. Temperatures of 1500° C. or less and preferably 1200° C. orless are preferred and temperatures between 600 and 1000° C. may be morepreferable. These low flame temperatures minimise decomposition of theceramic powder components during flame spraying. Moreover, they limitthe impact of evaporation of the materials to be flame sprayed and allowa deposition efficiency of more than 80%, i.e. more than 80% of thesolid mass originally introduced into the gun 12, becomes attached tothe substrate 19. Mechanically stable, scratch resistant flame sprayedcoatings are produced with these low temperatures.

The gun 12 is preferably held at 7 to 15 cm from the substrate 19 to becoated but this depends upon the size of the flame. Similar coatingshave been obtained using both oxy-acetylene flame spraying and plasmaspraying. Attention must be paid to the energy taken up by the sprayedparticles during the spraying and the transfer of this energy to thesubstrate. Intensive cooling of the substrate is preferred which may beon the side of the substrate remote from the deposited layer and/or onthe same side. By altering the velocity of the particles in the flame orplasma the dwell time therein may be altered, thus limiting the energyuptake by the particles.

The material of substrate 19 preferably has a high melt temperature anda high thermal conductivity and when the substrate 19 is to be used as atarget for a sputtering magnetron a good electrical conductivity ispreferable. It is also preferred if the thermal expansion of thesubstrate material is similar to that of the ceramic coating to beapplied. In accordance with embodiments of the present invention lowtemperature flame spraying and/or intense cooling of the substrate 19allows the use of substrates 19 with a thermal expansion coefficient upto at least twice or down to at least a half of the thermal expansioncoefficient of the ceramic coating. A non-limiting list of suitablematerials may be steel, iron, stainless steel, copper or copper alloys,however the low temperature flame spraying process in accordance withthe present invention, either independently or in combination withintense cryogenic cooling of the substrate 19, allows other materials tobe used such as paper, cardboard or polymeric materials. Preferably, thesubstrate 19 should be free of grease and dry before deposition.Preferably, the outer surface of metals is sand blasted and then lappedwith abrasive materials. Buffer layers between the substrate and thesprayed coating may be used such as Ni—Al or an In-alloy. These may beapplied by flame or plasma spraying before application of the metaloxide coating.

Where the substrate 19 is rigid it may be mounted in a suitable jig. Forexample, a cylindrical substrate 19 is preferably mounted in a rotatingdevice such as a lathe. The substrate 19 may be held by rotatable chucksat each end thereof. The temperature of the solidified flame sprayedcoating 40 on the surface of the substrate 19 is preferably measured bya temperature sensor 13, 15. The sensor head 13 is preferably a remotesensing optical head which is not in contact with the surface 40 of theflame sprayed coating. The temperature to be measured is of thesolidified coating 40 and not that of the coating immediately onimpacting the substrate 19 which may have a higher temperature. Hence,the temperature sensor 13 is preferably mounted so that it lags behindthe impact position of the flame sprayed materials a little. In additiona temperature sensor 31 may be provided inside the substrate 19 forfurther control of the deposition process. Control of depositiontemperature is an important aspect of the present invention. Control oftemperature affects the amount of thermal stress in the coating, a lowstress reducing the possibility of cracks forming in the coating.

In accordance with one embodiment of the present invention a means forintense cooling of the substrate 19 is provided. This is preferably acryogenic cooler comprising a supply 16 of cryogenic fluid and adelivery system 11, 14, 17, 29, 30. The delivery system may be adaptedto the form of the substrate 19. For example, for a cylindricalsubstrate 19 the cooling device may be a conduit 17 for supplying thecryogenic fluid to a control valve 11, a conduit 30 with regularlyspaced holes 29 for distribution of the cryogenic fluid inside thesubstrate 19 and a control device 14 for receiving the output of thetemperature sensor 13, 15 and for controlling the operation of thecontrol valve 11 so as to maintain the surface temperature of thesolidified coating 40 to within a certain range. Particularly preferredis a temperature range from room temperature (25 to 30° C.) to 150° C.and more preferably room temperature to 100° C. These low temperaturesavoid thermal stresses between the coating 40 and the substrate 19providing a good bond and good coating density, hardness and scratchresistance thus helping to ensure the long term stability of such acoating. Using a cryogenic fluid such as liquid nitrogen (77° K.) isquite advantageous and economical as it does not require thecomplication of perfectly sealed rotating inlets and outlets to thesubstrate 19 when water or other liquid coolants are used. Additionally,cryogenic fluids such as liquid nitrogen produce large temperaturegradients, thus increasing the thermal sink-effect. Other liquidcoolants such as water are not excluded from the present invention.

The cylindrical substrate 19 may be sealed by a seal 26 at one end andwith a rotating seal 28 at the other. The seal 28 may be provided with asealed feedthrough 27 for the supply of cryogenic fluid. If watercooling is used, rotating seals at both ends of the cylindricalsubstrate are considered very important to prevent escape of watervapour into the deposition environment. In accordance with an embodimentof the present invention it is preferred if the ends 26, 27 allow escapeof a cryogenic fluid which then forms a shield gas around substrate 19during the spraying process. Particularly preferred cryogenic fluids areliquid nitrogen, liquid oxygen and liquid air. With some complex ceramicmaterials, one or more components may be reduced in the sprayingprocess. For such materials it may be advantageous to use a shield gasincluding oxygen, e.g; liquid air or liquid oxygen, which may help toreoxidise the reduced component. On the other hand with other complexceramics it may be advantageous to reduce the contact time with oxygenat high temperatures, under which conditions liquid nitrogen would bepreferred, or a reducing gas may be included such as hydrogen. It ispreferable to control the atmosphere in the vicinity of the substrate 19during coating deposition to prevent the presence of excessive watervapour and in particular to prevent condensation of water on thesubstrate 19. This may be achieved by generally air conditioning the airaround the substrate 19 to reduce its dew point.

It is preferred if the deposition rate is selected in order to maintainthe substrate surface temperatures mentioned above. Assuming thecylindrical substrate as shown in FIG. 1, the rotation speed of thesubstrate 19, the linear speed of the gun 12 and the rate of materialexiting the gun 12 may be controlled to achieve the temperaturesspecified above. For instance, it has been found that when usingcylindrical substrates made of stainless steel of 15 cm diameter and upto 40 cm long, a powder delivery of 5-10 g/min was suitable to produce3-10 mm coatings when depositing a YBa₂Cu₃O₇ layer. The rotational speedof the substrate 19 may be in the range 10 to 100 RPM with a surfacespeed in the range 1 to 40 m/min and the longitudinal feed of the gun 12in the range 1-3 m/min, typically 2 m/min. The deposition rate perreciprocating pass of the gun 12 may be 10 to 50 micron thickness of thecoating. About 10% to 15% of the deposited coating had maintained thelattice structure of the powder and exhibited superconductingproperties. It will be appreciated by the skilled person that increasingthe deposition speed, deposition thickness per pass or the flametemperature or reducing the thermal conductivity of the substratematerial will increase the thermal load on the cooling system andadjustments of one or more of these parameters may be necessary toobtain satisfactory coatings. The thermal conductivity of the depositedmaterial is preferably between 1 and 5 Wm⁻¹K⁻¹. When deposited on asteel substrate the thermal conductivity preferably lies within therange 25 to 125 Wm⁻¹K⁻¹. These values are particularly preferred forYBa₂Cu₃O₇ coatings. Preferably, an adhesion promoter layer is appliedonto the substrate before application of the coating of the metal oxidecombination. The adhesion promoter may be a layer of Ni—Al or a layer ofan In-alloy, for example. The deposited coating is preferably impactresistant, e.g. withstands impact of a 0.036 kg steel ball from a heightof 2 meters. Preferably, about 20% or up to 30% of a noble metal isincluded in the oxide material to improve electrical and thermalproperties of the deposited layer. The noble metal is preferably silver.The noble metal may in included as a salt or oxide, e.g. silver nitrateor silver oxide, in the material to be sprayed. Preferably, theelectrical resistivity of the deposited layer is lower than 15×10⁻⁶Ohm.m, more preferably lower than 10×10⁻⁶ and most preferably less than5×10⁻⁶ Ohm.m. Values below 1×10⁻⁶ Ohm.m can be achieved. Up to 30% of anoble metal such as silver may be added to lower the resistivity. Thesevalues are particularly preferred for YBa₂Cu₃O₇ coatings.

FIG. 2 is a schematic representation of a further embodiment of theflame spraying process and apparatus in accordance with the presentinvention. Components in FIG. 2 with the same reference numbers as inFIG. 1 refer to equivalent items. The substrate 19 in accordance withthis embodiment is a foil or sheet of metal, plastic or other flexiblematerial which is wound from a pay-off spool 32 to a take-up spool 36.Where the final coating 40 cannot be spooled, the foil with coating 14may be drawn linearly from the pay-off spool 32 and cut into lengths.The coating 40, which may be a superconducting layer, is flame sprayedwith a flame spray gun 12 similar to the one described with respect toFIG. 1. In particular it is preferable to use a fuel with a lowercalorific value than acetylene such as natural or town gas, butane orpropane. Preferably, the temperature of the flame of the gun 12 impartsa temperature of 1500° C. or less, more preferably 1200° C. or less tothe material being sprayed through the flame. This material may be inthe form of powder either of finished components of the coating 40, e.g.oxides, or precursors thereof, e.g. nitrates, or may be in the form of aslurry of powders, e.g. oxides, or a solution, e.g. of nitrates. Gun 12may be controlled by hand or more preferably by a robot to providezigzag motions across the width of foil 19 thus applying an even coating40. Preferably a layer of 10 to 50 micron thickness is applied in eachpass.

The temperature of the coating 40 may be monitored by one or moreoptical sensors 13, 15. The temperature of the foil 19 is regulated bymeans of a cryogenic fluid supplied from a container 16 to a series ofholes or jets 29 via conduit 17, a controllable valve 11 and a conduit30. The valve 11 is controlled by a controller 14 to maintain thetemperature of the foil as determined by the sensor 13, 15 to less than400° C., preferably less than 150° C. and most preferably between 50 and100° C. Such low temperatures allow a wide range of materials forsubstrate 19 including polymeric materials, cellulosic materials as wellas metals. Although only one controller 14 is shown the presentinvention includes several controllers each with its own controllablecryogenic cooling device 11, 29, 30 for individually controlling thetemperature of different parts of the foil 19 or coating 40. Optionally,an optical encoder 34 may be attached to a roller 35. The opticalencoder may be read with an optical sensor 37, 38, the pulse frequencygenerated in the sensor 37, 38 being proportional to the linear speed ofthe foil 19. This value may also be used by the controller 14 to controlthe complete process to maintain the temperatures and coatingthicknesses mentioned above.

When producing superconducting coatings 40, it is preferred if there isno condensation of water onto the coating 40 nor onto the foil 19 so itis preferred if the atmosphere around the deposition equipment is airconditioned to reduce the dew point to below ambient temperature.Preferably the coated substrates in accordance with this invention arepreferably stored for long periods in a plastic bag filled with a dryinert gas such as dry nitrogen. One aspect of the present invention isthe flame spraying of powders which already have superconductingproperties in the powder form. Using the methods in accordance with thepresent invention it is possible to flame spray such coatings and retain10% to 15% of superconducting property of the coating 40 producedwithout extensive post-heat treatments.

The superconducting and/or ceramic powder and/or metallic powder to beused for flame spraying is preferably homogeneous, exhibits theappropriate rheological properties and correct stoichiometry to generatethe required properties in the final coating. Typical preferreddensities for superconducting powders may lie in the range 4 to 5 g/cm³.A non-limiting list of suitable materials which may be flame sprayed aspowders, slurries or liquid solutions in accordance with the presentinvention are: superconducting materials such as R¹Ba₂Cu₃O_(y) where Ris Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu; orBi_(2−x)Pb_(x)Sr₂Ca_(n−1)Cu_(n)O_(y), Tl₂Ba₂Ca_(n−1)Cu_(n)O_(2n+3),HgBa₂Ca_(n−1 Cu) _(n)O_(2n+2+δ); or Ba₂Ca_(n−1)O_(2n+1), orCaBa₂Ca_(n−1)Cu_(n)O_(2n+δ); or cuprate high temperature superconductorsof the general formula A_(m)E₂R_(n−1)Cu_(n)O_(2n+m+2) where A, E, R areselected from various cations such as A=Bi, Tl, Hg, Pb, Cu or alanthanide element, E=Ba or Sr and R=Ca or rare earth element; orpiezo-electric ceramics, for example, with the general formulaM(Zr_(x)Ti_(1−x))O₃ where M=Pb, Ba or Sr; or refractory ceramic oxides,nitrides, carbides or phosphates, e.g. Al₂O₃, MgO, Zr_(x)O_(y); ormetals and their alloys.

In accordance with a further embodiment of the present invention amethod is provided for production of suitable ceramic powders. Bystarting from aqueous solutions containing the salts of the metals inthe correct proportions a reactive precursor powder can be obtainedusing commercially available spray drying equipment in batches ofkilograms. The type of salt (mostly nitrates) should preferably becompatible with thermal decomposition to oxides in further processessuch as sintering or flame spraying. In accordance with the presentinvention spray dried nitrate powders may be flame sprayed directly orthe powders may first be sintered and then flame sprayed.

A spray drying system 50 in accordance with an embodiment of the presentinvention for the delivery of powder suitable for subsequent flamespraying is shown schematically in FIG. 3. The input liquid is drawnfrom a suitable source 53 via a peristaltic pump 54 to a spray head 71.Pressurised air 51 is drawn in through an air dryer and optionalpre-heater 52 to the spray head 71 by a suction device such as a fan 63at the end of the generally closed system. The liquid from source 53enters the spray head 71 which is cooled by any suitable means 55 toprevent clogging due to early evaporation of the liquid. The liquid isatomised in a co-current two fluid nozzle 71 by the dry pressurised air51 and discharged it into a chamber 56 where it dries to a powder. Theliquid from source 53 may be a solution of suitable nitrates or a slurnyof the relevant oxides with the addition of other agents such asbinders.

Air 65 is drawn in by fan 63 over a heater 64 and introduced intochamber 56 via a ring orifice 72 which surrounds the outlet of the sprayhead 71. The air 65 also heats the spray head 71. The circumferentialair flow 65 guides the evaporating liquid in chamber 56 and helps toprevent the powder sticking to the walls of the chamber 56. The airthroughput of fan 63 is chosen so that powder of the correct grain sizeis swept from chamber 36 through an optional heater section 58 into apowder collector 59. Heavier particles settle out in trap 57 and areremoved from the bottom of chamber 56.

The powder collector 59 may be any suitable device such as a cyclone, abag filter or an electrostatic filter although a cyclone is preferred.The cyclone discharges the powder into a removable container 60 sealedto the bottom of the cyclone 59. Spent air is removed via the trap 61and scrubbed in scrubber 12 to remove impurities. The final clean air isexhausted to atmosphere by the fan 63 which controls air flow throughthe system.

The control system 66-70 for the process functions as follows. Therotational speed of the centrifugal air pump 53, the temperature of theheating element 64 and the flow of the atomised air are set withcontroller 67, 70. Air flow is measured by gauge 68. The temperature ofthe hot air 65 and the air in the tube leading from the chamber 56 tothe optional heater 58 is monitored using thermocouples 66, whereasfinal powder temperature is monitored by temperature sensor 69.

After spray drying, the powder may be sintered in a single step. Forexample, to produce a superconducting powder of the general formulaYBa₂Cu₃O₇ with optional Ag, the required nitrates are dissolved in waterin the correct stoichiometric proportions and spray dried as indicatedabove. The nitrates are then reduced to oxides by sintering at 920-960°C. for 40 to 60 hours. Optionally the nitrates may first be reduced byheating in air at 780° C. for 10 hours before sintering at the abovetemperatures and times. The YBa₂Cu₃O₇ powder produced by this procedureis superconducting. On aspect of the present invention is to producepowders with superconducting properties by spray drying and optionalsintering and then to flame spray these superconductive powders at thelowest flame temperatures necessary to obtain melting of the powder andcoating formation on the substrate while cooling the coating in thefastest possible way. By this procedure the crystal structure present inthe superconducting powder is disturbed as little as possible by theflame spraying process. Of course, melting the powder during flamespraying causes complete loss of crystal organisation if the time in themelt is long. By lowering the flame temperature and shortening the timein the melt phase by cooling the coating very rapidly in accordance withthe present invention, some local crystal organisation is kept in thefinal flame sprayed coating, e.g. about 10% of the final coating is inthe superconducting phase, thus providing a coating on the substratewith an optimum starting condition for further heat processing todevelop full superconducting properties. The addition of the metallicsilver enhances the thermal and mechanical properties in later flamespraying and magnetron sputtering.

Alternatively, the powder for flame spraying may be spray dried fromslurries of the relevant oxides in the correct stoichiometricproportions with the optional addition of silver in the above apparatusin accordance with the present invention. For instance, in themanufacture of a ceramic material the mixture of oxides may be producedby individually sieving them to 40 micron and then mixing in the correctproportions to obtain the stoichiometric proportions in the finalcoating. A quantity of deionised water is added of about 60% by weightof the powder as well as a quantity of an organic binder such as PVA(polyvinyl acetate) equal to about 2% by weight of the powder and thenmixed into a slurry. The slurry is then spray dried as described aboveresulting in powder with a grain size from 30 to 50 micron. Generally,spray dried oxide slurries do not require sintering before flamespraying. The organic binder may be burnt out during flame spraying orin a special sintering step.

Spray drying of 10% by weight nitrate solutions generally produce grainsizes of 3 micron on average with at least 90% of the grains between 0.5and 5 microns. In order to obtain the required grain size it ispreferable to sinter as mentioned above. Light grinding and sieving ofthis sintered powder may produce a mass fraction of more than 80% withgrain sizes between 40 and 80 micron. By the variation of appropriateconcentrations of the solution of the aqueous media 53, and/or theaddition of binders and/or the spray drying of slurries rather thansolutions, allows control of the grain size in the final powder tobetween 2 and 100 microns. For example, the present invention includesthe addition of organic binders such as polyvinyl acetate (PVA) to theliquid to be spray dried to control grain size in the final powder. Suchbinders may be burnt out in a later high temperature process such assintering. An average grain size of 40 to 80 microns is preferred forgood flame spray deposition. The final powder may be lightly milled andsieved to be improve the homogeneity of grain sizes.

One aspect of the present invention is the inclusion of silver metal inthe final superconducting ceramic coating. This is achieved as mentionedabove by inclusion of about 20% to 30% by weight of the ceramicmaterials of silver nitrate when nitrate solutions are spray dried andthe flame sprayed or by addition of Ag₂O powder in an oxide slurry whichis then spray dried and flame sprayed. The addition of silver in theflame sprayed material is beneficial for the inter-grain adhesion andheat dispersal during flame spraying thus yielding a strong and densecoating. The silver improves the thermal and electrical conductivity ofthe flame sprayed coating which is beneficial to the sputtering processwhen the substrate is used as a sputtering target. The improvedconductivities allow higher power throughput for the magnetron thantargets not containing silver.

The flame spraying process in accordance with the present inventionallows the reconditioning of targets for sputtering magnetrons. It iswell known that the presence of a static race-track plasma on amagnetron target during sputtering results in an erosion groove and poortarget utilisation. Using the flame spraying process of the presentinvention such a worn target may be reconditioned by spraying theappropriate target material into the erosion groove and building up thetarget to its former thickness in these regions. By providing theintensive cryogenic cooling described above, the general targettemperature may be kept below 400° C., preferably below 150° C. and mostpreferably between room temperature (˜25° C.) and 100° C. These lowtemperatures result in little damage to the existing target materialwhile still providing a mechanically strong coating in the old erosiongrooves. Such as process is particularly economic when the targetmaterial is expensive such as superconducting materials. The flamespraying gun 12 described above may be hand held and the contour of theerosion groove in the used target followed building up the lost materialslowly, preferably 10 to 50 micron per pass. Preferably the gun 12 iscontrolled by a robot which is programmed to execute the correct motionswith the gun 12 to fill up the erosion groove in the target.

While the invention has been shown and described with reference topreferred embodiments, it will be understood by those skilled in the artthat various changes or modifications in form and detail may be madewithout departing from the scope and spirit of this invention as definedin the attached claims.

What is claimed is:
 1. A method of depositing by flame or plasmaspraying at atmospheric pressure a layer onto a substrate, the layerhaving a thickness of at last 5 mm, the coating comprising metal oxides,the method including the steps of: depositing an additional noble metalwith the coating to increase thermal conductivity of the coating, andduring deposition of the coating, cooling the substrate so that thesolidified coating thereon has a temperature between 25 and 150° C. 2.The method of claim 1, wherein the temperature of the solidified coatingduring deposition is held at between 50 and 100° C.
 3. The methodaccording to claim 1, wherein the noble metal is silver.
 4. The methodaccording to claim 3, wherein silver containing material is included inmaterial to be sprayed to result in up to 30% silver in the coating asdeposited.
 5. The method according to claim 1, wherein the spraying stepincludes spraying a material through a spraying head, the material beingin the form of one of a powder, a slurry or a solution.
 6. The methodaccording to claim 1, wherein the cooling is with a cryogenic liquid. 7.The method according to claim 1, wherein the coating comprises asuperconductive precursor and at least 10% of the layer is in asuperconductive phase is deposited.
 8. The method according to claim 3,wherein the layer has a thickness of greater than 8 mm.
 9. A compositecomprising: a substrate and a coating obtained by the method of claim 1,the thickness of the coating being at least 5 mm, the coating comprisingmetal oxides and the deposited coating comprising the addition of anoble metal to increase thermal conductivity of the coating.